1. Field of the Invention
The present invention generally relates to an antenna apparatus used in a portable communication apparatus. More specifically, the present invention is directed to a structure of an antenna apparatus capable of producing desirable antenna radiation patterns without modifying the antenna structure.
2. Description of Prior Art
As is well known in the field, an electromagnetic radiation pattern of an antenna would be varied when a conductive article would be located adjacent to this antenna, since a high-frequency current may flow through the antenna during transmission/reception of electromagnetic waves.
Therefore, to obtain a desirable radiation pattern of the antenna, effect of the conductive member in the vicinity of the antenna should be taken into consideration. For example, in the portable communication apparatus, since a circuit board is provided with a grounding conductive layer of a comparatively large surface area, the effect of such grounding conductive layer should be taken into consideration. In recent, to protect the circuit boards from external electromagnetic effect, portable communication apparatus are provided with an electromagnetic shield plate or such circuit boards are installed within a metal housing. But, in the portable communication apparatus, attention should be paid to effects of the electromagnetic shield plate and the metal housing.
FIG. 1 is a view illustrating a structure of antenna apparatus which has been proposed recently. The antenna apparatus draws much attention in the art, since the antenna apparatus is effective to obtain a desirable radiation pattern, and is often used for a communication apparatus of a type in which the circuit board is electromagnetically shielded with a metal housing.
As shown in FIG. 1, the antenna apparatus is composed of an antenna 1 (a so called .lambda./4 monopole antenna) having a length of one fourth of a wave length and a metal housing 2 formed with a notch 3 in the side wall thereof. The notch 3 is formed with an opening end 3a in the side wall thereof. The notch 3 is formed in the side wall of the metal housing at a position apart by a length of .lambda./4, i.e., a length of 0.25.lambda. from the upper surface on which an electric supplying point 1a is provided. The notch 3 has a depth of 0.25.lambda., and the ceiling and bottom composing the notch 3 are connected by an end wall (the leftend wall as viewed in FIG. 1). Therefore, the notch 3 has a stub function.
Then, the portion defined from the uppermost portion of the right side surface of the metal housing to the opening end 3a of the notch 3, namely the portion having the length of 0.25.lambda. will be cooperated with the .lambda./4 monopole antenna 1, which are therefore operated like a .lambda./2 dipole antenna.
The above-described conventional antenna apparatus requires the notch 3 having the depth of 0.25.lambda. (wavelengths). As a result, the horizontal (transverse) width 1 of the metal housing 2 necessarily becomes longer than 0.25.lambda., which may impede compactness of the metal housing 2.
As to the manufacturing stages of the conventional antenna apparatus, when another antenna apparatus is manufactured which is operable in another frequency different from that of the above-described conventional antenna apparatus by changing the length of the above-described .lambda./4 monopole antenna 1, a length from an upper surface of a metal housing to an opening end should be varied in order to be fitted to this new frequency. As a consequence, there are drawbacks in the conventional antenna apparatuses that various metal housings whose notch forming positions are different from each other should be manufactured, depending upon the frequencies of the electromagnetic waves used in the communications.